Offshore structures (e.g. floating production and drilling platforms, SPAR platforms, floating storage and offloading vessels) are typically held in place in the ocean through use of anchoring or mooring lines. One end of the lines is attached to chain stoppers typically mounted on the offshore structures while the other end of the lines is attached to anchors or anchoring piles that are embedded into the seabed. Such an anchoring configuration allows the offshore structures to stabilize and secure themselves in an operating location amid the harsh, unpredictable weather and environmental conditions generally experienced in the ocean.
In some offshore structures (e.g. SPAR platforms that may operate in Arctic conditions), supporting columns of the offshore structures are designed with circumferential recesses at sections that are approximately located at or near to the sea level. The reduced cross-sectional area of the supporting columns at those sections assists the offshore structures in “cutting” through oncoming waves or ice chunks, thereby mitigating the effects of any undesirable loading forces that might otherwise be transmitted therefrom to the offshore structures. However, the circumferential recesses also expose the chains of the mooring lines (refer to FIG. 1) which are typically guided down along the supporting columns to chain fairleads attached thereto at or below the sea level.
Consequently, the exposure of the chains poses several problems. Firstly, the exposed chains might result in accumulation or encourage formation of unwanted articles such as ice chunks around the recessed sections, thereby inadvertently increasing the cross-sectional area of the recessed sections. In addition, there is also a greater chance and risk of severing the chains should a boat accidentally steer into the supporting columns which might then endanger the lives of the crew on the offshore structure.
U.S. Pat. No. 7,377,225 B2 discloses a spar-type platform which includes a hull defining a centerwell extending downward to a keel. The hull includes a reduced diameter neck portion for diverting ice flow. Adjustable ballast tanks allow the hull to be moved between a ballasted down position defining an upper water line, and a ballasted up position defined by a lower water line. A riser a support buoy is disposed in the keel. Risers extend through the centerwell, each having an upper portion extending upward from the support buoy and a lower portion supported in the support buoy. A disconnect system detachably connects the support buoy to the hull and the upper portion of each riser to the lower portion thereof, whereby the hull and the upper portion of each riser are selectively detachable from the buoy and the lower portion of each riser for movement to avoid a collision with a floating object. The disconnect system comprises a remotely operable riser coupler that releasably couples the upper portion of each riser to the lower portion thereof, a latch mechanism that is remotely-operable to releasably secure the buoy to the keel of the hull.
U.S. Pat. No. 7,197,999 B2 discloses a spar-type offshore platform which includes a buoyant upper hull structure supporting a deck and having lower end in which is received a buoyant lower mooring module. The upper hull structure is connected to the mooring module by connection lines. The upper hull structure is removed from the mooring module by disconnecting the connection lines from the upper hull structure while leaving the connection lines attached to the mooring module and while the mooring module remains moored to the seabed.
U.S. Publication No. 2008/0311804 A1 illustrates a system, for use at offshore locations of large depth, for mooring a production vessel or floating unit at a location over a hydrocarbon reservoir and for connecting risers that can be carrying hydrocarbons up from the sea floor to a production vessel that stores the hydrocarbons, flowlines for water injection, gas lift, gas export, umbilicals and mooring lines that moor the vessel. Both the mooring lines and the risers are disconnectably connected to the vessel through a connection buoy, or connector.
U.S. Pat. No. 7,510,452 B2 illustrates a disconnectable mooring system for a vessel comprises a mooring buoy member and a turret structure mounted in a moonpool of the vessel. The mooring buoy member is anchored to the seabed and has a plurality of passages each adapted to receive a riser. The turret structure has a receptacle for receiving the buoy member and a locking device for locking the buoy member in the receptacle.